1. Field of the Invention
The present invention is generally related to a magnetic sensor that utilizes a magnetically sensitive component in combination with a permanent magnet to sense the presence or absence of a ferromagnetic object in a detection zone and, more particularly, to a magnetic sensor that incorporates a permanent magnet which has a beveled surface formed at one end of the magnet.
2. Description of the Prior Art
Many different types of magnetic sensors are known to those skilled in the art. Certain magnetic sensors utilize a permanent magnet to provide a bias magnetic field that is distorted when a ferromagnetic object moves through a preselected detection zone. The distortion of the magnetic field is sensed by a magnetically sensitive component which provides an output signal that changes to indicate the presence or absence of the ferromagnetic object within the detection zone. A common application of this type of sensor is in a geartooth sensor used in automotive applications. Sensors of this type can be used in the timing apparatus of an automobile engine and, alternatively, in conjunction with automatic braking systems.
U.S. Pat. No. 4,520,311, which issued to Petr et al on May 28, 1985, discloses a current to pulse-sequence transducer for transforming a current to be measured into a sequence of pulses which has a certain mark-to-space ratio. The transducer includes a first transforming device for transforming the current to be measured into a measurement magnetic field, a second transforming device for generating a reference magnetic field and magnetic field comparison device including a magnetoresistive thin film comparator normally providing a zero output when the fields are equal, but generating an impulse when the strength of one magnetic field exceeds that of the other. In addition, the device comprises an impulse processing device. The impulse processing device includes a differentiator amplifier connected to the output of the magnetoresistive thin film comparator and a Schmitt trigger circuit postcoupled to the differentiator amplifier.
U.S. Pat. No. 4,524,932, which issued to Bodziak on Jun. 25, 1985, describes a railroad car wheel detector using a Hall effect element. The element is incorporated into an integrated circuit unit with temperature compensation. It also includes voltage regulation and amplification and is mounted on top of a permanent magnet made of ceramic material with the critical Hall axis aligned with the magnet pole axis. In order to avoid saturating the Hall cell, it is positioned within a pole-to-pole hole in the magnet which creates a flux null space.
U.S. Pat. No. 4,401,944, which issued to Narimatsu et al on Aug. 30, 1983 discloses a displacement detector having first and second magnetoresistive elements with a bias field at 45 degrees to each element. The apparatus includes first and second magnetoresistive elements having respective angularly disposed current path portions through which a bias current flows and to which a bias magnetic field is supplied. An external magnetic signal field is supplied from a magnetic source and the magnetoresistive elements are relatively displaced with respect to the source. The bias field is supplied at an angle of approximately 45 degrees with respect to the direction in which the bias current flows in each current path. The source of external magnetic field generates a signal field whose polarity gradually decreases and then changes over to an opposite polarity at a boundary region.
U.S. Pat. No. 5,477,143 which was issued on Dec. 19, 1995 to Wu and assigned to the Assignee of the present application, discloses a proximity sensor that is provided with two magnetoresistive elements that are disposed in a common plane and displaced from a lateral surface of a permanent magnet. The common sensing plane of the magnetoresistive elements extends in a direction generally parallel to a magnetic axis of a permanent magnet that extends between the north and south poles of the magnet. A detection zone is defined relative to a preselected magnetic pole face and the magnetoresistive elements provide first and second signals that can be compared to define a third signal which is representative of the presence or absence of the magnetically permeable object within the detection zone. The magnetoresistive elements can each comprise a plurality of magnetoresistors which are arranged in a Wheatstone bridge configuration for the purpose of providing the first and second signals described above.
Regardless of the specific type of magnetic sensor used, certain characteristics are important to the operation of the sensor. One of the most important characteristics of a magnetic sensor is the distinctiveness of its output signal with regard to the presence and absence of a ferromagnetic object in the detection zone. For example, a very slight change in the magnitude of the sensor's output signal could possible create difficulty in the precise identification of the leading edge of the ferromagnetic object as it moves past the face of the sensor. In many automotive applications, it is necessary for the sensor to be able to accurately and precisely identify the location of the ferromagnetic object as it moves through the detection zone. In common applications of magnetic sensors that are used as geartooth sensors, the ferromagnetic objects that move through the detection zone are the teeth of a rotatable gear. It is therefore advantageous if a magnetic sensor can be provided with certain operational characteristics that enhance its ability to perform these detection functions.
It would be beneficial if a sensor could be provided with the characteristic of switching its output at precisely the same target angular position even though there may exist variation in the relative position between the sensor assembly and the target. This variation in the relative positions of the target and the sensor result from the normal variations that exist because of manufacturing tolerances. For example, automobile engines are not identical to each other regardless of the efforts made to achieve this goal. In addition, there exists slight dimensional differences between individual sensors regardless of the manufacturing tolerances used during production. In addition, the relative positions of the sensor and the target can change because of variations in temperature and the effects of thermal expansion. The variation in the air gap between a geartooth sensor and a rotatable target is typically between 0.10 millimeters and 1.5 millimeters.